Conventional thermal infrared sensors have an infrared absorber, typically formed at a surface of a semiconductor substrate. The absorber absorbs an incoming infrared flux, converting the incident flux into heat. The temperature rise produced by this heat causes a thermally-induced change in a thermal sensing element such as a thermistor that is mounted on or in proximity to the absorber. The change is generally converted into an electrical signal and the signal is delivered to a readout circuit.
In such a sensor, temperature resolution can be increased by increasing the efficiency with which the incident flux is absorbed or increasing the response of the thermal sensing element to a fixed temperature change. For example, if the absorption of an infrared flux causes a change in a resistance value, a large change in resistance for a small absorbed power is desired.
With reference to FIGS. 18-19, a conventional thermal infrared sensor 100 has a micro-bridge structure that defines a space M between an infrared absorber 100A and a semiconductor substrate 101. The space M reduces heat conduction between the infrared absorber 100A and the semiconductor substrate 101, thereby increasing the response of the sensor. The infrared sensor 100 includes an infrared absorption layer and a thermoelectric layer formed on the infrared absorber 100A (these layers are not shown in FIGS. 18-19). The thermoelectric layer produces an electrical change in response to a temperature change. Two bridges 100B support the absorber 100A and suspend the absorber 100A above the substrate 101. The bridges 100B include electrical connections that connect the thermoelectric layer with a diffusion layer 101A that serves as a readout electrode. The micro-bridge structure lowers the thermal conductivity from the absorber 100A to the semiconductor substrate 101; such a sensor is preferable to an infrared sensor in which the absorber is in direct contact with a substrate. Therefore, the infrared sensor 100 has an increased response to infrared radiation.
Micro-bridge structures for infrared sensors have been developed in which the bridges 100B are longer and have smaller cross-sectional areas, reducing thermal conductivity further. Improvements in response can also be achieved by using materials with low heat capacities for the absorber 100A. Because only the absorber 100A absorbs the incident flux, the absorber 100A should occupy as large a proportion of the sensor surface area as possible. The proportion of the sensor surface area occupied by the absorber 100A is referred to as the "aperture ratio."
In addition, infrared imaging devices that have arrays of infrared sensors such as the sensor 100 have been developed. In these imaging devices, the proportion of the device surface area occupied by all the absorbers 100A (this proportion is also referred to as the "aperture ratio") must be large to achieve improved temperature resolution. Because the sensors 100 of an imaging device are separated by isolation bands that do not detect incident radiation, it is especially important that the absorption area of the sensors 100 be large.
In one prior-art imaging sensor, a readout circuit is situated directly below the absorber 100A in order to increase the aperture ratio. However, it is difficult to make an array of sensors having micro-bridges and maintain a large aperture ratio without simultaneously increasing sensor heat capacity.
If a fixed ratio between the heat capacity and the heat conductivity from the absorber 100A is maintained, changes in the aperture ratio do not effect sensor response. Consequently, a specified temperature resolution can be achieved by varying the size of the sensor. In this case, temperature resolution is not limited by sensor aperture ratio, but by practical limits in sensor fabrication. The bridges 100B are to be thin, narrow, and long to reduce heat conduction, and the absorber 100A surface area is to be as large as the bridges 100A can support. Therefore, fabrication of the bridges is difficult, especially for small sensors.
In a conventional sensor such as the sensor 100, the surface area of the sensor 100 must provide some area for the bridges 100B and contacts 100C. Thus, potential increases in aperture ratio are limited. It is even more difficult to achieve a large aperture ratio in an imaging device comprising an array of such sensors.
With reference to FIG. 19, the contacts 100C and the bridges 100B of the sensor 100 support the absorber 100A above the substrate 101. A cross-section of the contacts 100C is a V-shape, narrow where the contacts 100C attach to the substrate and wider where the contacts 100C connect to the bridges 100B. As a result, a large surface area is occupied by the contacts 100C. For example, a typical sensor 100 has a width of 40-50 .mu.m and a space M having a height h of 2.5 .mu.m. If the support that connects the contacts 100C to the substrate 101 has a diameter R1=1.0 .mu.m, and if the contacts 100C have a 45.degree. taper, then the diameter R for the contacts 100C must be about 6 .mu.m. Because flux incident upon the contacts 100C and the bridges 100B produces very little heating, a large surface area occupied by the contacts 100C and the bridges 100B reduces the aperture ratio of the sensor 100, thereby degrading the responsivity of the sensor.
With reference to FIG. 19, a space L is defined between the bridges 100B and the absorber 100A. The space L thermally isolates the bridges 100B and the absorber 100A but reduces the aperture ratio. Imaging systems comprising an array of sensors such as the sensors 100 also have an isolation region 101B defined between adjacent sensors 100. The isolation regions 101B prevent transmission of heat on one sensor 100 to neighboring sensors, but also reduce the aperture ratio. Infrared imaging devices using conventional sensors have aperture ratios of less than about 50 percent, i.e. more than one-half of the area of the substrate 1 is unavailable for the absorption of the incident flux.
Thus, when the aperture ratio is low, a single thermal infrared sensor 100 must occupy a large area in order to improve the temperature resolution of the sensor. It is difficult to integrate such large sensors into an imaging device. If the number of sensors required for an imaging device is specified and the size of the substrate 1 is limited, then sensors having a small aperture ratio use only a small portion of the substrate area for absorption.